The development of a GC-electron-capture detection instrument to determine accurately certain important trace C 1 halocarbons in a single analytical procedure from ambient air is reported. The procedure utilizes preconcentration at room temperature on an efficient microtrap filled with a commercially available adsorbent, Carboxen, followed by direct thermal desorption in a single stage on to a high-resolution capillary column. Detection is achieved with dual electron-capture detectors in series; the second being oxygen doped to dramatically enhance the sensitivity of the detector towards those halocarbons (hydrofluorochlorocarbon 22, CH 3 Cl) which normally react feebly with thermal electrons.